Highly efficient operation of a social infrastructure system for producing and distributing energy, water, gas, oil and the like has become increasingly important to realize a society capable of sustainable development. The highly efficient operation of the social infrastructure system will be achieved by making apparatuses included in the system work with high efficiency. This requires an apparatus monitoring and controlling network capable of highly efficient operation.
A promising technique for the highly efficient apparatus operation is estimation/forecasts of working conditions of an apparatus based on a wide range of data obtained from many sensors arranged around the apparatus.
A network is used to gather and analyze data from many sensors, to estimate the working conditions of an apparatus and to send control information to the apparatus. In this network, the number of transmission paths from the sensors is extraordinarily large. Against this background, therefore, it is desirable that the network be built using wireless technologies instead of conventional wired technologies.
The social infrastructure system provides the society with lifelines. For this reason, even in a case where specific ones of transmission paths in the network naturally fail or are artificially interfered with, the social infrastructure needs to fulfill the function of providing appropriate lifelines by performing communications while avoiding failed or obstructed transmission paths.
In general, in radio communications, transmission paths are in an open space, and multiple transmission paths are automatically created between a transmitting point and a receiving point. In general, meanwhile, communications are transmitted and received in batches. For these reasons, when specific transmission paths naturally fail or are artificially interfered with, the communication quality of the network deteriorates to a large extent, and may even go as far as to ruin the network.
On the other hand, wired communications allow a third party to identify transmission paths. When a third party finds and intrudes into a transmission path, lifeline supply will be seriously affected. This possibility cannot be denied.
In other words, neither wireless nor wired communications using the conventional techniques have yet achieved a network which is highly robust against man-made or natural failure and interference in specific transmission paths.
To sensors installed in an apparatus included in the social infrastructure system and actuators for controlling the apparatus, the apparatus itself works as an electromagnetic wave scatterer. In a radio network using an electromagnetic wave as a communication medium, therefore, it cannot be expected that a radio device included in this network performs communications along a line of sight. The radio network is thus operated in an abnormal environment which uses a wave not traveling in a line of sight, that is to say, a multi-reflected wave resulting from reflections by apparatuses.
An electromagnetic wave is a vector wave. Reflection causes a unique change in the radio wave's physical actual condition, so-called “polarization” perpendicular to the traveling direction. Radio waves with the same polarization automatically emitted in multiple directions from a transmitter are uniquely reflected by multiple apparatuses, and arrive at a receiver as radio waves with a unique change in the polarization, through multiple propagation paths.
As a result, the receiver is forced to use an unpredictable polarization direction which results from synthesizing the vectors of the arriving radio waves. Because of symmetry between the transmission and the reception in radio communications using an electromagnetic wave, an unpredictable change in the polarization direction caused between the transmission and the reception is unique to the specific pair of transmitter and receiver, and the polarization direction irregularly changes at every moment as the radio wave environment changes.
The radio device is required to perform transmission and reception appropriately even in this unpredictably and irregularly changing radio wave environment.
A solution to this problem is provided by Patent Literature 1.
Patent Literature 1 discloses a polarized wave diversity transmission system. Patent Literature 1 states as follows (see Abstract): “PROBLEM TO BE SOLVED: To provide a polarized wave diversity transmission system for obtaining large fading improvement effect in a mobile communication, etc. SOLUTION: A transmitter 1 sends a radio wave which is circular polarized wave or 45° or 135° polarized wave from an antenna 2. The radio wave is made incident on vertical and horizontal reception antennas 4 and 5 and orthogonally polarized components are received, respectively. Their receive signals are supplied to receivers 6 and 7 and a diversity reception circuit 8 selects one of the outputs of the receivers or combines them together, so that a reception output circuit 9 obtains the output of the polarized wave diversity reception”.
As described above, the technique disclosed in Patent Literature 1 deals with the problem using the communication system in which: the transmitter sends an electromagnetic wave with a rotating polarization which is temporally different; and the receiver receives two polarized waves fixedly orthogonal to each other.